The Ames Limestone is the boundary and limestone stratum between the Casselman and Glenshaw Formations within the Conemaugh Group. This limestone can be found throughout Western Pennsylvanian, Eastern Ohio, and northwestern West Virginia. It is a laterally extensive stratum that contains what many call the best fossils in Western Pennsylvania. Being limestone, the fossils within are naturally mostly if not all marine fossils. An exception to this would be if a land-based creature were to fall into the sea and be buried, which is very rare.

Geologic Age

The Upper Freeport coal bed is the lower boundary of the Glenshaw Formation while the Ames Limestone is the upper boundary. The Ames marks the boundary between the younger Gzhelian, the last stage in the Upper Pennsylvanian, and the older Kasimovian. While many older papers reference names such as Missourian or Virgilian Stages, the International Commission on Stratigraphy uses standard global names.

Global StageAgeTime Represented
Gzhelian4.8 MY298.9 – 303.7 MYA
Kasimovian3.3 MY303.7 – 307.0 MYA
Global geologic time stages and their approximate ages.

The Ames is shown as the yellow to orange merge points on the map below. Published in 1980, this map is by the Commonwealth of Pennsylvania Department of Environmental Resources. One interesting detail about this map is that the freeway portion of PA 28 terminates shortly after passing the Freeport area. This shows the age of the map, published about 40 years ago.

Map showing the Casselman (orange), Glenshaw (yellow), Allegheny (green), and Monongahela Group (Tan) formations.
Key for Map
Key for the above map

The Ames preserves fossils dating back to the Carboniferous Period. More specifically, the late Pennsylvanian. One of my goals in local exploration is to find sources for Ames limestone. So far I have found none, but according to the maps, there are many places to search.

Here is a map of the Casselman Formation embedded on Google Maps.

In a 1933 report by H.H. Hughes, he reported the Ames Limestone to be nearly 30 inches thick near Perrysville, which I believe was in Bell Township, South East of Vandergrift. In the Plum, Lower Burrell area it was 16-35 inches thick and completely absent at two drill sites. Fragments found in the Northern part of the Freeport Quadrangle, South Buffalo Township showed thicknesses of 3 to 4 inches.

Compared to the Brush Creek Limestone

The Brush Creek Limestone is a much darker matrix, likely due to a more anoxic environment when the sea invaded the land. A lack of oxygen means more of the biomass from the living creatures above doesn’t break down at the bottom of the sea. The Brush Creek is an older limestone, with a separation of approximately 2.3 million years of time between the two ancient seas.

Brush Creek Limestone next to Ames Limestone
Brush Creek Limestone (Left) next to Ames Limestone (Right). Notice the lighter color of the Ames. Both contain many fossils.

Exploring Ames Limestone

The first place I got to see the limestone up close was in Frick Park. While there are several locations in which to find it, I have yet to find one with exposed rocks. I’ve done online searches for Ames Limestone and have often landed on the Sedimentation in Western Pennsylvania page, an old Pitt Geology web resource.

Following their directions, I quickly found two sets of outcrops. I could not find the hard ledge in the stream bed as mentioned. However, it appears there has been a lot of erosion in the stream bed, and there were several blocks of either sandstone or limestone along the creek.

I did not bring a hammer or chisel. Quickly I learned that you are not allowed to work on rocks in Frick Park. However, it was nice to finally see it up close.

  • Ames Limestone in Frick Park
    Ames Limestone as Ledge
  • Ames Limestone in Frick Park
    Ames Limestone Showing Fossils

The Ames Limestone in Gilpin Township

Ames Limestone in Gilpin Township, Pennsylvania
What appears to be a straight run of Ames Limestone in Gilpin Township. The shale below the limestone is eroded while the natural cement characteristics of limestone enable it to resist erosion longer.

The limestone cut above is a few miles from one of the termination Northern fingers of Ames limestone that stretch North East along the rolling hills of the Allegheny Plateau. The farthest Northern mapped portion of the Ames exists a bit past DuBois, PA in Penfield, PA. This is over 60 miles from Parks Township.

While the Casselman layer may be eroded away, it does not necessarily mean the Ames Limestone is gone. The stone remains long after the silty base layer and more is eroded. Float limestone can be found in local streams, still around after the more erodible layers have been washed away.

Ames Limestone along Route 28.

There are several places the Ames outcrops along Pennsylvania Route 28. During the construction of the road, massive amounts of the landscape were removed, exposing many hidden rocks. It can be seen more or less along stretches from Etna to Tarentum.

  • Ames Limestone along PA 28 North
  • Ames Limestone along PA 28 North
Ames Limestone as seen along PA 28 North, just past the Pittsburgh Mills exit.

More Information about the Ames

References